1. Field of the Invention
The invention relates to a continuous process for the preparation of phenol by the direct oxidation of benzene with hydrogen peroxide, carried out in a fixed bed reactor.
More specifically, the invention relates to a process for the preparation of phenol wherein the oxidation reaction is carried out in the presence of a catalyst based on titanium silicalite and under particular operative conditions.
The invention also relates to a process for the activation of the catalyst based on titanium silicalite TS-1.
2. Description of the Related Art
Phenol is an industrial intermediate of great importance, which is widely used in the production of polycarbonates or other phenolic resins.
Phenol is currently produced according to the “Hock process”, which contemplates the alkylation of benzene to cumene and subsequent oxidation of cumene to hydroperoxide, which decomposes to phenol and acetone.
Various processes are known in the art for the preparation of phenol, which are based on the direct oxidation of benzene with hydrogen peroxide, in the presence of suitable catalytic systems.
A process is known, for example, which is carried out in the presence of a catalyst based on titanium silicalite and in an organic solvent capable of enhancing the contact between the organic substrate and hydrogen peroxide.
The conversion and selectivity of processes for the preparation of phenol by direct oxidation can be improved by operating in the presence of specific solvents, such as sulfolane, for example (EP A 919531).
In this case, the process is carried out in a batch-type reactor operating in a two-phase reaction system consisting of the solid catalyst and an organic phase comprising sulfolane/water/benzene in such a ratio as to make the reaction mixture homogeneous.
Improvements in the productivity of processes for the production of phenol can also be obtained by activation of the catalyst with hydrogen peroxide and fluorine ions, as described in European patent application EP A 958861.
Further improvements can be obtained by operating in a three-phase reaction system consisting of a solid catalyst, an aqueous phase and an organic phase comprising the aromatic compound and the solvent, as described in patent application PCT/EP02/12169.
The processes for the preparation of phenol by the direct oxidation of benzene described in the known art are generally carried out in CSTR-type reactors in which the catalyst based on titanium silicalite is kept in suspension in the form of a fine powder.
In these processes, the separation of the reaction effluent from the catalyst can only be effectively carried out by filtration inside the reactor.
This operation, however, cannot be effected in the processes of the known art because, as a result of the low concentrations of the products, the filtrating surface necessary would be too high.
The only possible technological solution consequently consists in effecting the filtration outside the CSTR reactors. This, however, is technologically complex due to the necessity of using both additional equipment and abrasion-resistant materials and also of maintaining the efficiency of the filters.